import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        int count = frogCount(n);
        System.out.println(count);
    }

    private static int frogCount(int n) {
        if(n == 2) return 2;
        if(n == 1) return 1;
        if(n < 1) return 0;

        int frog1 = frogCount(n - 1);
        int frog2 = frogCount(n - 2);

        return frog1 + frog2;
    }
}